The Geological Society of America Memoir 212 2015
Simulating foreland basin response to mountain belt kinematics and climate change in the Eastern Cordillera and Subandes: An analysis of the Chaco foreland basin in southern Bolivia
Todd M. Engelder Jon D. Pelletier* Department of Geosciences, University of Arizona, 1040 E. 4th Street, Tucson, Arizona 85721, USA
ABSTRACT
The relative importance of crustal thickening, lithospheric delamination, and cli- mate change in driving surface uplift and the associated changes in accommodation space and depositional facies in the adjacent foreland basin in the central Andes has been a topic of vigorous debate over the past decade. Interpretation of structural, geochemical, geomorphic, and geobiologic fi eld data has led to two proposed end- member Tertiary surface uplift scenarios for the Eastern Cordillera and Subandes in the vicinity of the Bolivian orocline. A “gradual uplift” model proposes that the rate of surface uplift has been relatively steady since deformation propagated into the Eastern Cordillera during the late Eocene. In this scenario, the mean elevation of the region was >2 km above mean sea level (msl) by the late Miocene or earlier. Alternatively, a “rapid uplift” model suggests that the mean elevation of the Alti- plano was <1 km above msl, and the peaks of the Eastern Cordillera were more than 2 km below their modern elevations until rapid uplift began ca. 10 Ma. Determin- ing which of these uplift scenarios is most consistent with the stratigraphic record is complicated by the potentially confounding effects of global climate changes and lithospheric delamination in the stratigraphic record. In this study, we use a coupled mountain-belt–sediment-transport model to predict the foreland basin stratigraphic response to these end-member surface uplift scenarios. Our model results indicate that the location and height of the migrating deformation front play the dominant roles in controlling changes in accommodation space and grain size within the fore- land basin. Changes in accommodation space and rates of sediment supply related to climate change and lithospheric delamination play secondary roles. Our results support the conclusion that the Eastern Cordillera likely gained most of its modern elevation prior to 10 Ma, in contrast with recent proposals that most of the modern elevation was obtained during the late Miocene. This conclusion is consistent with the most comprehensive paleoaltimetric analysis of the region to date.
Engelder, T.M., and Pelletier, J.D., 2015, Simulating foreland basin response to mountain belt kinematics and climate change in the Eastern Cordillera and Sub- andes: An analysis of the Chaco foreland basin in southern Bolivia, in DeCelles, P.G., Ducea, M.N., Carrapa, B., and Kapp, P.A., eds., Geodynamics of a Cordil- leran Orogenic System: The Central Andes of Argentina and Northern Chile: Geological Society of America Memoir 212, p. 337–357, doi:10.1130/2015.1212(17). For permission to copy, contact [email protected]. © 2014 The Geological Society of America. All rights reserved.
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INTRODUCTION 2011b) as described further in the Discussion section. At a larger scale, the Andes as a whole have been an invaluable natural labo- Sediments eroded from mountain belts are primarily depos- ratory for exploring feedbacks between climate and rock uplift ited in foreland basins, which are elongate troughs created by (Montgomery et al., 2001; Strecker et al., 2007). fl exural loading of the lithosphere adjacent to mountain belts An unresolved issue for the central Andes is: When did the (DeCelles and Giles, 1996). Foreland basin stratigraphy can pro- topography of the central Andes rise to its modern elevation? One vide useful data for reconstructing climate and crustal deforma- model for the topographic development of the central Andes near tion histories within ancient mountain belts (e.g., Heller et al., the latitude of the Bolivian orocline posits that the mean topog- 1988; DeCelles et al., 1998; Marzo and Steel, 2000; Uba et al., raphy reached near-modern elevation following Neogene crustal 2007). The fl uvial systems that transport sediments from eroding thickening within the Subandean zone; thus, earlier crustal thick- mountain belts are sensitive to changes in sediment supply, dis- ening within the Eastern Cordillera was not suffi cient for the east- charge, and sediment accommodation rates, and thus, the foreland ern margin of the central Andes to rise to near-modern elevations basin stratigraphy in these systems is a partial record of changes (Isacks, 1988; Gubbels et al., 1993). In addition to crustal thick- in climate and the geometry of the mountain belt load through ening, continental lithospheric foundering has been proposed as a time. The fl exural profi le of foreland basin accommodation space mechanism for the generating at least some of the rapid Neogene depends on the rigidity of the lithosphere that is underthrusted surface uplift posited by this model (Kay and Kay, 1993; Garzi- beneath the mountain belt as well as the width and elevation of one et al., 2008; DeCelles et al., 2009). Continental lithospheric the mountain belt load. If the foreland basin geometry and rigid- foundering involves removal of negatively buoyant lower crust ity of the underthrusted lithosphere can be constrained, then it is (i.e., eclogite root) and mantle lithosphere by either delamination possible to infer changes in the mountain belt load that occurred or Rayleigh-Taylor instability. Initial paleoelevation and geomor- during the development of the foreland basin. As such, numer- phic studies did support late Miocene rapid surface uplift of the ous studies have inferred tectonic histories for ancient mountain region (Ghosh et al., 2006; Hoke et al., 2007; Garzione et al., belts by fi tting foreland basin geometries predicted by a coupled 2008). Pedogenic carbonate and carbonate cement samples col- mountain belt and foreland basin numerical model to observed lected between 17°S and 18°S within the Altiplano and Eastern foreland basin isopach data (e.g., Toth et al., 1996; Ford et al., Cordillera Neogene stratigraphy contain decreasing δ18O values 1999; Garcia-Castellanos et al., 2002; Prezzi et al., 2009). in progressively younger units (Garzione et al., 2008). Garzione The eastern margin of the central Andes (i.e., the portion of et al. (2008) interpreted this oxygen-isotope trend as evidence for the mountain belt that is to the east of the Altiplano Plateau) in an increase in elevation of the Eastern Cordillera of 2.5 ± 1 km southern Bolivia is an ideal region for constraining late Cenozoic from 10 to 7 Ma. Although these results are in agreement with changes in mountain belt geometry and climate through numeri- fossil leaf and clumped isotope data collected from the Altiplano cal modeling of foreland basin stratigraphy. The sediments pre- and Eastern Cordillera (e.g., Gregory-Wodzicki et al., 1998; served in the foreland basin have been well documented, and Ghosh et al., 2006), the elevation gain predicted by all three fi eld and laboratory constraints have been reported in the liter- methods can be complicated (i.e., biased toward larger values) by ature for shortening and exhumation rates in the hinterland for climate change due to the uplift of the Andes (Ehlers and Poulsen, the period of time over which the eastern margin of the central 2009). Also, recent studies have interpreted the change in Mio- Andes has developed. A W-E transect of the Cenozoic foreland cene oxygen isotopes to be the result of regional climate change basin stratigraphy is exposed within the retroarc fold-and-thrust caused by uplift of an already signifi cantly elevated (≥2 km) belt. Detailed isopach maps and stratigraphic sections for the central Andes above an orographic threshold (Poulsen et al., Cenozoic foreland basin stratigraphy have been developed based 2010; Insel et al., 2012). Most recently, Quade et al. (this volume) on a combination of measured sections from the fold-and-thrust showed that, while rapid uplift may have occurred locally in the belt and correlations between well-log data and two-dimensional Eastern Cordillera as documented by Garzione et al. (2008), the (2-D) seismic data (Sempere et al., 1997; DeCelles and Horton, easternmost Puna and Eastern Cordillera rose gradually to >3 km 2003; Echavarria et al., 2003; Uba et al., 2005, 2006). In addi- by no later than 15 Ma. Evidence for late Miocene rock uplift is tion to data from the foreland basin, the timing and magnitude of recorded by paleosurfaces that exist on both the eastern and west- shortening and exhumation within the mountain belt have been ern margins of the central Andes. Barke and Lamb (2006) esti- constrained by fi eld mapping and thermochronology (McQuar- mated 1.7 ± 0.7 km of localized rock uplift for the San Juan Del rie, 2002; Muller et al., 2002; Oncken et al., 2006; Ege et al., Oro surface of the Eastern Cordillera and Interandean tectono- 2007; Barnes et al., 2008, 2012). While most of the papers in morphic regions since 12–9 Ma, when the surface was abandoned this volume focus on the NW Argentina transect through the cen- and incised. The Barke and Lamb (2006) results do not constrain tral Andes, we focus here on the Bolivian transect due to these the magnitude of surface uplift, however. The difference between unusually rich data sets. We further assume that the two transects rock and surface uplift is particularly signifi cant because local- (Bolivia and NW Argentina) behave in a broadly similar way. ized rock uplift can be much higher than mean regional surface This assumption is supported by basin evolution and thermochro- uplift due to isostatic effects (England and Molnar, 1990). We nologic studies (e.g., DeCelles et al., 2011; Carrapa et al., 2011a, will refer to this conceptual model for surface uplift as the rapid
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uplift end-member model. Although Quade et al. (this volume) annual precipitation should increase sediment supply through appear to have disproven this model, it is still important to test enhanced erosion rates and increase the transport capacity of this model using other lines of evidence (besides paleoaltimetry). foreland basin fl uvial systems via greater mean annual discharge. A second end-member model for the topographic evolution Lower-crustal delamination is another mechanism that has of the central Andes invokes gradual surface uplift since the late been proposed for controlling the late Cenozoic foreland basin Eocene, when deformation propagated from the Western Cordil- stratigraphy in the central Andes. The cordilleran cycle, a con- lera into the Eastern Cordillera. Evidence for pre-Neogene defor- ceptual model for mountain belt development and cyclicity pro- mation comes from Eocene exhumation ages within the Eastern posed by DeCelles et al. (2009), posits that as lower crust and Cordillera and changes in paleocurrent directions within Paleo- mantle lithosphere are underthrust beneath a growing mountain gene stratigraphy of the Altiplano and Eastern Cordillera (Horton belt, magmatic and petrologic processes lead to the formation of et al., 2002; McQuarrie et al., 2005; Ege et al., 2007; Barnes et a dense eclogitic root that acts as a subsurface negatively buoy- al., 2008). Although pre-Neogene shortening is considered low ant load, lowering the elevation of the mountain belt relative to (<100–150 km) for building a high-elevation plateau, long-term a state of isostatic equilibrium (DeCelles et al., 2009; Pelletier shortening should have led to crustal thickening and isostatic et al., 2010). If suffi cient shortening takes place, the eclogite rebound in the Eastern Cordillera. Therefore, unless erosion rates root reaches a critical thickness or volume and is delaminated or exceeded or were equal to rock uplift rates, the Eastern Cordil- removed via a Rayleigh-Taylor instability. This cordilleran cycle, lera should have been rising (i.e., surface uplift as defi ned by as posited by DeCelles et al. (2009), includes episodic periods of England and Molnar, 1990) since the late Eocene, barring some modest (i.e., 0.5–1 km) increases in surface uplift and shortening mechanism for keeping the elevation of the Andes low during an rates, both of which could have had a signifi cant effect on the extended period of deformation. The gradual uplift end-member adjacent foreland basin through the modifi cation of both rates model posits that the Andes gained the majority of its modern of sediment supply and creation of accommodation space. Pres- elevation prior to ca. 10 Ma. ently, the Puna Plateau is thought to be in a postdelamination The Cenozoic stratigraphy of the Chaco foreland basin in state in this conceptual model (Schurr et al., 2006; DeCelles et Bolivia shows an increase in both grain size and sedimentation al., 2009) and, as a consequence, has a mean elevation that is rates during the late Miocene (Uba et al., 2006, 2007). This strati- signifi cantly higher than the Altiplano Plateau. The Altiplano, in graphic change could have been the result of rapid surface uplift. turn, is thought to have had a delamination event at 10 Ma (Kay However, previous studies have inferred that this depositional et al., 1994). DeCelles et al. (2009) proposed that the Altiplano trend might instead be primarily controlled by distance from the may already be in an early stage of a new cycle, and its lower approaching fold-and-thrust belt (DeCelles and Horton, 2003; elevation is a consequence of newly forming eclogite loads. Evi- Uba et al., 2006), which provides the topographic load to drive dence for delamination beneath the Altiplano comes from seis- fl exural subsidence and accommodation space creation. Thus, the mic velocity analysis of the eastern Altiplano lithosphere at 20°S primary mechanism that caused these changes in late Miocene (Beck and Zandt, 2002). A low-velocity zone occurs within the stratigraphy is still uncertain. In addition to thrust belt migration, upper mantle beneath the thickened crust of the eastern Altiplano climate change and continental lithosphere foundering have also and western edge of the Eastern Cordillera and is interpreted as been emphasized as potential controls on the foreland basin stra- a location where the cold-fast upper mantle has been removed tigraphy of the central Andes in recent years. For example, Klein- by a delamination event. Might stratigraphic trends in the late ert and Strecker (2001) documented a change from previously Miocene Chaco foreland basin deposits in Bolivia be a signature dry to wetter conditions in the Santa Maria Basin of northern of lithospheric foundering instead of climate change or fold-and- Argentina between 9 and 7 Ma. Based on climate model stud- thrust belt propagation? ies, Poulsen et al. (2010) and Insel et al. (2012) have shown that Previous studies have numerically modeled the evolution of surface uplift of a signifi cantly elevated central Andes in the late the central Andes as a coupled mountain-belt–foreland-basin sys- Miocene would have caused an increase in precipitation along tem. Flemings and Jordan (1989) fi rst simulated the rapid uplift the eastern front of the central Andes. Uba et al. (2007) found end-member model for the last 5 m.y. of deformation in the central a signifi cant (i.e., factor of 5) increase in time-averaged deposi- Andes with a two-dimensional model. They concluded that the tional rates within the stratigraphy of the Subandean thrust belt in foreland basin adjacent to the central Andes should have shifted Bolivia at ca. 8–7 Ma based on U-Pb dating of tuffs within Mio- from narrow and underfi lled to broad and overfi lled as sediment cene volcanic rocks. Coeval with changes in depositional rates supply outpaced sediment accommodation. However, the results and climate conditions within the foreland depositional facies of of their model are potentially limited by the fact the mountain the Cenozoic fl uvial units, a shift occurs from single-thread sinu- belt component of their model was simplifi ed by assuming a ous channels to more amalgamated alluvial megafan facies (Uba constant topographic slope and sediment supply through time. et al., 2006). Uba et al. (2007) interpreted these changes in depo- Prezzi et al. (2009) recently applied a more rigorous deforma- sitional rates and facies to be a consequence of a change from tion and erosion model to test the effect of mountain load geom- semiarid to more humid climate conditions during the intensi- etry and elastic thickness of the South American lithosphere on fi cation of the South American monsoon. An increase in mean the sediment accommodation rates within the foreland basin of
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the central Andes since the middle Miocene. They found that by signal (e.g., unconformity, grain size change) of climate change decreasing the elastic thicknesses beneath the Eastern Cordillera or continental lithospheric delamination recorded in the upper and Interandean zones between 14 and 6 Ma and by deforming a Miocene Chaco foreland basin stratigraphy. detailed structural cross section developed by McQuarrie (2002), their model results adequately fi t modern gravity anomalies and GEOLOGIC BACKGROUND isopach distributions for the late Cenozoic stratigraphy recently described by Uba et al. (2006) in the Subandean zone of Bolivia. The central Andes of southern Bolivia contain a high- In this study, we explore the linkages among thrust-belt elevation, internally draining, low-relief plateau. This portion of kinematics, climate change, continental lithosphere delamina- the Andes also has the highest magnitude of total shortening (i.e., tion, and the foreland basin stratigraphy in the central Andes ~285 km of minimum crustal shortening within the Eastern Cor- using a coupled, two-dimensional numerical model. Flemings dillera, Interandean, and Subandean zones; Isacks, 1988; McQuar- and Jordan (1989) simulated the foreland response to a fi xed top- rie, 2002; Oncken et al., 2006). The orogenic belt is generally ographic slope and constant sediment supply and, thus, did not broken up into tectonomorphic regions, which include (from west capture the effects of feedbacks among mountain belt erosion, to east): the Western Cordillera, Altiplano, Eastern Cordillera, sediment supply to the foreland basin, and sediment accommo- Interandean, Subandean, and Chaco foreland basin zones (Fig. 1). dation within the foreland basin. The Prezzi et al. (2009) study The modern topographic divide between westward internal drain- focused on the changes in foreland basin accommodation based age into the Altiplano basin and eastward drainage into the Chaco on surface uplift due to a very specifi c history of crustal defor- foreland basin resides within the Eastern Cordillera, a bivergent mation and changes in elastic thickness under constant climate fold-and-thrust belt. Thrusts within the Eastern Cordillera detach conditions. Both studies focused on crustal thickening as the in Ordovician-aged horizons and predominantly exhume Paleo- dominant mechanism for driving foreland basin development. In zoic through Mesozoic units (McQuarrie, 2002). Further east, the contrast, in this paper, we aim to constrain the relationship of Interandean and Subandean zones are where deformation is cur- the Cenozoic foreland basin stratigraphy to end-member surface rently active. These zones contain dominantly eastward-verging uplift models, climate change, and lower-crustal delamination in imbricate thrusts that generally exhume younger stratigraphic order to place fi rmer constraints on the paleoelevation history of units compared to the Eastern Cordillera. The modern deforma- central Andes. First, we determine which end-member surface tion front is located between 0.5 and 1 km above mean sea level uplift model is most consistent with the available stratigraphic (msl). Beyond the Subandean zone, the Chaco foreland basin and tectonic data for the development of the eastern margin of extends an additional 250–600 km east into Bolivia, Paraguay, the central Andes. Second, we determine whether there is some and the Pantanal wetlands of Brazil until it onlaps Precambrian
Figure 1. Digital elevation model for the central Andes displaying the tectonomorphic zones after Barnes et al. (2008). Topography is from the Shuttle Radar Topography Mission (SRTM) 90 m data set. The tectonomorphic regions are the following: WC—Western Cordillera; AL—Altiplano; EC—Eastern Cordillera; IA—Interandean zone; SA— Subandean zone. The thick white lines are thrust faults located on the boundary between major divisions, and the thin white lines mark political boundaries.
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basement highs in eastern Bolivia and southwestern Brazil (Hor- belt, and thicknesses that are low, considering the amount of ton and DeCelles, 1997). An exception occurs between 19°S and time contained within the units. The Yecua Formation is unique 20°S, where the foredeep basin deposits pinch out onto the Alto to the Subandean zone Cenozoic units because it also contains de Izozog basement high over a distance that is less than 200 km shallow-marine facies in addition to fl uvial and lacustrine facies from the deformation front (Uba et al., 2006). in outcrops located north of 20°S. Moving up through the stratig- The Chaco foreland basin stratigraphy has been documented raphy into the Tariquía, Guandacay, and Emborozú Formations, within the exposed thrust sheets of the Eastern Cordillera, Inter- overall grain size increases from that of the Yecua Formation, andean, and Subandean zones by DeCelles and Horton (2003), paleocurrent data indicate a shift to transport from the west, and Echavarria et al. (2003), Horton (2005), and Uba et al. (2005). depositional rates increase. Based on dating volcanic ash layers Isopach trends have also been developed for the buried modern in the Cenozoic foreland basin stratigraphy, there is a factor of foreland basin units based on correlations between well data and fi ve increase in depositional rates between the Yecua and Tariquía seismic cross sections (Uba et al., 2006). Starting at the base of Formations (Uba et al., 2007). Higher in the stratigraphic section, the Cenozoic stratigraphy from the Subandean zone (Fig. 2), the depositional rates decrease by half upward into the Guandacay Petaca and Yecua Formations contain fl uvial deposits with well- Formation. Although deposition is generally conformable within developed paleosols, paleocurrent trends that dominantly show the Cenozoic stratigraphy, unconformities bound the Petaca For- transport toward or along strike with the approaching mountain mation, and an angular unconformity separates the Guandacay
Age (Ma) Emborozu 0.0 500-2000 m Guandacay Tariquia 500-1500 m 1200-3800 m Yecua 10.0 50-600 m
Mondragon ? ~500 m 20.0 Petaca ? Figure 2. Generalized stratigraphic col- 10-250 m umns for the Cenozoic stratigraphy of the Chaco foreland basin after DeCelles Carmargo and Horton (2003) and Uba et al. (2006, >2000 m ? 2007). Formation thicknesses are dis- 30.0 played beneath the formation names. The question marks represent uncer- msc tainty in the boundaries between forma- Subandean zone tions due to a lack of absolute age data. ? A detailed discussion of the age uncer- 40.0 tainties for the Cenozoic stratigraphy of Cayara the Eastern Cordillera, Interandean, and ~190 m Subandean zones can be found in De- ? Celles and Horton (2003) and Uba et al. (2006). m—mudstone; s—sandstone; Impora c—conglomerate. 50.0 ~60 m ?
60.0 Santa Lucia ~100 m Paleocene Eocene Oligocene Miocene msc Eastern Cordillera and Interandean zone
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and Emborozú Formations. The upper unconformity of the Pe - TABLE 1. TIME-AVERAGED SHORTENING RATES taca Formation has been interpreted to record the uplift and pas- USED IN EACH MODEL SIMULATION sage of the forebulge as the Subandean zone passed from the Zone Shortening rate Duration (m/yr) (m.y.) back bulge into the foredeep basin (Uba et al., 2006). In contrast, Altiplano 0.0014 30 the angular unconformity located between the Guandacay and Eastern Cordillera back thrust 0.0015 38 Emborozú Formations has been interpreted as the initiation of Eastern Cordillera forethrust 0.0022 30 wedge-top deposition in the Subandean zone. Low-angle uncon- Interandean 0.0044 22 Western Subandes 0.0037 18 formities have been documented at the base of coarsening-upward Eastern Subandes 0.0067 10 cycles within the age-equivalent stratigraphy of the Tariquía and Guandacay Formations in northern Argentina (Echavarria et al., 2003). However, cycles and signifi cant unconformities were not documented in the Tariquía and Guandacay Formations of the tain belt with sediment transport and fl exure within a depo- southern Bolivia Subandean zone (Uba et al., 2006). Farther sitional foreland basin. A 2-D model is valid for this region west and older in age, similar stratigraphic trends occur within of the Andes because the mountain front is nearly linear, and the remnant foreland basin stratigraphy of the Eastern Cordillera paleofl ow directions for the majority of the Cenozoic foreland and Interandean zones (Fig. 2). The transition from the Cayara basin stratigraphy are perpendicular to the mountain front. The into the Camargo Formation involves an overall increase in grain topography of the numerical model represents the mean topog- size and changes in paleocurrent direction and apparent deposi- raphy of the eastern margin of the central Andes (i.e., western tional rates. One stratigraphic difference between this remnant edge of the model begins in the Altiplano) located between foreland basin and the Neogene foreland basin of the Subandean 18°S and 21°S. In our kinematic model, shortening and rock zone is that an unconformity does not bound the lowest Cenozoic uplift rates are prescribed. Although time-averaged shortening unit in the section. Overall, the most signifi cant trends within the rates (Table 1) can be directly calculated from published val- Cenozoic foreland basin stratigraphy of the eastern margin of the ues and used as preliminary input into the model, obtaining central Andes are the apparent increase in depositional rates and valid rock uplift rates is an iterative process of running simu- grain size with decreasing age, and thus, any numerical models lations, checking the results against independent published for the foreland basin evolution of the eastern margin of the cen- data (e.g., modern topography and exhumation magnitudes), tral Andes must honor these trends. and adjusting the rock uplift rates within each tectonomorphic zone (Fig. 3). In this paper, we report model outcomes that are MODEL DESCRIPTION consistent with published data for the central Andes, including the modern topography, shortening rates from balance cross Summary sections, exhumation magnitudes, Cenozoic isopach data, and modern basin geometries. The numerical model of this paper is a 2-D kinematic All model simulations begin at 43 Ma. Evidence from fi eld model that couples an actively deforming and eroding moun- mapping and thermochronology suggests that deformation did
ESA A B ESA 6.0 6.0
- 4 WSA - 4 WSA 4.0 4.0 IA IA
FT FT 2.0 BT 2.0 BT AP AP Rock Uplift Rate (10 m/yr) Rock Rock Uplift Rate (10 m/yr) Rock 0.0 0.0 40.0 30.0 20.0 10.0 0.0 40.0 30.0 20.0 10.0 0.0 Time (m.y.) Time (m.y.)
Figure 3. The distribution of prescribed zonal rock uplift rates within the eastern margin of the central Andes hinterland through time for both the gradual (A) and rapid (B) end-member uplift models. Symbols for each zone are the following: triangles—Altiplano (AP); dashed line—Eastern Cordillera back thrust (BT); dashed dotted-line—Eastern Cordillera fore- thrust (FT); dotted line—Interandean (IA); asterisk—western Subandean (WSA); solid line—eastern Subandean (ESA).
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not propagate into the Eastern Cordillera until the late Eocene, contributing drainage basin area (m2), x is the lateral distance and, therefore, starting the model before then is unnecessary from the model origin (m), l is the distance along the principal and would have been poorly constrained with available data. channel from the headwaters of the drainage basin (m), S is The older foreland basin section in the region was interpreted by the local channel slope (unitless), and m and n are constants DeCelles and Horton (2003) to represent distal foredeep to fore- that determine the dependence of local erosion on discharge bulge deposits and only represents a partial foreland basin pro- and channel slope. In this standard 2-D implementation of the fi le. Having a more complete foreland basin profi le that includes stream-power model, h represents the longitudinal profi le of the foredeep is necessary to constrain the topography of the adja- the main channel of a three-dimensional (3-D) river basin. cent mountain belt. The usual approach in such cases (e.g., Whipple and Tucker, 1999) is to assume that contributing area goes as the square of Rock Deformation distance from the drainage divide. This assumption implicitly adds tributary area to the main channel in a manner that is Dynamic models for crustal deformation (e.g., Simpson, statistically consistent with the way in which tributaries add 2004) that directly solve for visco-plastic deformation within a area in real 3-D river basins. In Equation 1, we assume that the fold-and-thrust belt could be applied to the central Andes. This length of the principal channel is proportional to the square approach, however, comes with the drawbacks of longer com- root of the contributing drainage area (Hack, 1957). We also putational times and the diffi culty of calibrating the model to assume that the area and slope exponents m and n have val- multiple types of calibration data. We therefore take a simpler ues of 0.5 and 1, respectively. Evidence from theory and fi eld approach in this paper. In the model, the mountain belt is par- studies predicts that the ratio of m/n is near 0.5 (Whipple titioned into individual blocks for each tectonomorphic region and Tucker, 1999). Although the slope exponent n can range (i.e., Altiplano, Eastern Cordillera, etc.). Exceptions are made between 0.66 and 2.0 depending on the relationship between for the Eastern Cordillera and Subandean zones. The Eastern slope and stream power (shear stress), we chose to make the Cordillera, a bivergent fold-and-thrust belt, was divided into stream power linearly proportional to the slope (n = 1.0 and separate back-thrust and fore-thrust blocks because these two m = 0.5), consistent with the assumption of many other studies regions were rapidly exhumed at different periods of time (e.g., Kirby and Whipple, 2001; Snyder et al., 2000). Bedrock (Barnes et al., 2008). The Subandean zone was divided into incision rates for active mountain belts are on the order of a western and eastern block to allow the central and eastern 0.1–1.0 mm/yr (Montgomery and Brandon, 2002). When m/n
Subandean zone to continue to subside until deformation prop- is equal to 1/2, we found that ke must be on the order of agated into the region during the late Miocene. Deformation 10−6 (m/yr) to appropriately reproduce the range of calculated is simulated by uniform rock uplift and shortening applied to exhumation rates (i.e., 0.1–0.6 mm/yr) for time scales of 102 each actively deforming block. Time-averaged shortening rates to 107 yr for the central Andes (Safran et al., 2005; Ege et al., were determined for input to the model by dividing the total 2007; Barnes et al., 2008). The boundary between the bedrock shortening determined from fi eld-based balanced cross sections portion of the model (where stream-power–driven erosion for each tectonomorphic zone by the duration of fault activity of bedrock is modeled) and the alluvial portion of the model (McQuarrie et al., 2005; Muller et al., 2002). The positions of (where grain-size–dependent sediment transport is modeled) the model nodes located to the east of the deformation front is allowed to fl uctuate in the model. If a given model node are fi xed, while the model nodes to the west of the deformation contains sediment or sediment is deposited (i.e., upstream front are allowed to translate toward the foreland basin as if rid- sediment supply exceeds the transport capacity), then the ing over a décollement. The total propagation of the deforma- node is treated as an alluvial channel, and no bedrock incision tion front (and hence the forebulge) is ~536 km, which is close occurs. Otherwise, bedrock incision occurs, and the amount of to the ~600 km of total propagation of the forebulge since the newly created sediment transported downstream is controlled late Eocene estimated by DeCelles and Horton (2003). by the transport capacity.
Bedrock Channel Erosion Sediment Transport
A stream-power model is used to determine bedrock inci- Sediment transport of a bimodal grain-size distribution sion rates within the mountain belt (Howard and Kerby, 1983; (i.e., gravel and sand) is simulated with a modifi ed version of Whipple and Tucker, 1999): the diffusion model approach, which states that sediment fl ux is proportional to local channel slope. Linear slope-dependent n sediment fl ux, when combined with conservation of mass, ∂h ∂h = −k Am = −k A1/ 2 S = −k lS , (1) gives a diffusion equation for the evolution of the longitudi- ∂ e ∂ e e t x nal profi le of the foreland basin (Paola et al., 1992). In the
where h is elevation above msl (m), t is time (yr), ke is the model of this paper, we added a threshold slope term to the bedrock erodibility (yr–1 for n = 1.0 and m = 0.5), A is the diffusion equation model because transport of gravel requires
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a threshold slope to initiate transport. The equation for alluvial for the displacement of a thin elastic beam subjected to a spa- deposition and erosion along a channel profi le in our model is tially distributed vertical load (Turcotte and Schubert, 2002): a combination of mass balance (i.e., the Exner equation) and ∂ 4 w()x the slope-dependent sediment-transport equation: D + (ρ − ρ )gw()x = L(x), (4) ∂ 4 m s x q = k ()S − ϕ , S >ϕ s g , (2) where w is the defl ection of Earth’s crust (m), D is the fl exural q = 0 , S ≤ϕ ρ 3 ρ s rigidity (Nm), m is the density of the mantle (kg/m ), s is the density of the mountain crust or foreland sediment (kg/m3), g is the acceleration due to gravity (m/s2), and L(x) is the topographic ∂h ∂q q − q = − s = s, upstream s, downstream , (3) load (kg/ms2). For each simulation, except for those involving ∂ ∂ ∂ t x x eclogite root delamination (discussed later), we solve for the defl ection due to a topographic load every time step using a Fou- 2 ϕ where kg is the transport coeffi cient (m /yr), is the threshold rier transform method (Watts, 2001). Viscous relaxation effects
slope, which is a function of grain size, and qs is the local sedi- were not considered in the model because we interpret strati- ment fl ux (m2/yr). The transport coeffi cients for gravel and sand graphic patterns over geologic time scales that are greater than are determined from a relationship that depends on discharge relaxation time scales. and river type (Paola et al., 1992). Paola et al. (1992) calculated Several studies have calculated the fl exural rigidity of the the values for braided and meandering rivers to range between central Andes using 2-D methods (Horton and DeCelles, 1997; 1.0 and 7.0 × 104 m2/yr for a drainage basin with a length of Stewart and Watts, 1997; Tassara, 2005). Their results suggested 100 km and a mean annual precipitation of 1 m. We chose a that fl exural rigidities along the eastern margin of the central value of 1.0 × 104 m2/yr for the gravel transport coeffi cient, Andes range between 1.5 × 1023 and 4.0 × 1024 Nm. We chose to which is a reasonable value for braided, gravel-dominated use a fl exural parameter of 150 km because this value best fi ts the streams with relatively small drainage areas. The transport observed modern basin geometry between 18°S and 20°S. This coeffi cient for sand was chosen to be ~6.5 × 104 m2/yr, which value is consistent with the results of Chase et al. (2009), who is on the order of the transport coeffi cient used by Flemings found that the fl exural parameter should be less than 220 km for and Jordan (1989) for their simulation of the central Andes. The the central Andes. The fl exural parameter is defi ned as the fol- threshold slope for gravel entrainment is ~0.001, which rep- lowing (Turcotte and Schubert, 2002): resents effective channel parameters of a median grain size of 1/ 4 ⎡ 4D ⎤ 0.02 m and bankfull fl ow depth of 1.85 m. The chosen median α = ⎢ ⎥ , (5) ⎣()ρ − ρ g ⎦ grain size is on order with the median grain size of bed-load m s material observed within the modern Rio Pilcomayo located in the eastern Subandean zone (Mugnier et al., 2006). where α is the fl exural parameter (km). Rearranging Equation Channel armoring is an emergent property of the model 5 and applying values from Table 2 yields a fl exural rigidity of obtained by incorporating a threshold slope for gravel trans- ~6.8 × 1023 Nm, which is well within the range of calculated port. If discharge is insufficient to entrain gravel present on effective fl exural rigidities for the central Andes. Although pre- the channel bed during a time step, then the finer-grained vious studies have suggested that the elastic thickness varies in sand deposited beneath the gravel is prevented from being space and time (e.g., Toth et al., 1996; Prezzi et al., 2009), such transported during that time step. This process should variations are diffi cult to constrain. Therefore, our model imple- reduce both alluvial and bedrock erosion in regions of lower ments a uniform and constant elastic thickness. regional slope where gravel is present. Active bed material layers in natural channels are on the order of 1–2 grains thick (Hassan et al., 2006). Although our active layer is on TABLE 2. PARAMETERS USED the order of a meter, it is valid to have a larger active layer IN END-MEMBER MODEL SIMULATIONS for the purposes of this study because we are focusing on −6 ke (m/yr) 1.0 × 10 5 long-term (>10 yr) grain-size trends that average over many 2 4 kg (m /yr) 1.0 × 10 individual flooding events. 2 4 ks (m /yr) 6.5 × 10 φ 0.001 Flexural Isostasy α (km) 150 ρ (kg/m3) 2750 The bedrock and alluvial surface dynamics models are s ρ 3 coupled to a fl exural foreland basin in order to quantitatively m (kg/m ) 3300 ρ 3 assess accommodation space creation and the migration of the e (kg/m ) 3600 2 forebulge through time in the model. The fl exural model solves g (m/s ) 9.8
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Numerical Methods 7 Ma; Molnar and Garzione, 2007; Garzione et al., 2008). We assume that the majority of the foundering time is spent growing We ran three types of experiments using our numerical the instability in the lower crust–mantle interface by the initial model: (1) an end-member surface uplift model experiment, factor of e due to the high resistance to fl ow of the mantle when (2) an eclogite root foundering experiment, and (3) a climate minimal perturbations in the lower crust–mantle interface exist. change experiment. The end-member surface uplift model Later, when a signifi cant pressure gradient exists at the base experiment simulates the last 43 m.y. when deformation is con- of the eclogite layer due to the formation of a drip, the pinch- centrated in the eastern margin of the central Andes. The pur- ing and foundering portion of the drip event occurs much more τ pose of this experiment is to contrast the foreland basin response rapidly. Based on this argument, we inferred that a is approxi- (e.g., changes in depositional rates and grain size) to a mountain mately less than or equal to 3 m.y. Using densities of 3300 and belt that is gradually uplifting through time against the foreland 3600 kg/m3 for upper mantle and eclogite and a root growth basin response to rapid surface uplift caused by Neogene crustal period of 3 m.y., the range in maximum eclogite layer thickness thickening. The parameters used in this experiment are found in is calculated to be between 1.87 and 46.88 km as the effective Table 2. During the simulations of this experiment, mean annual viscosity of the underlying mantle layer varies between 4.0 × precipitation rates were held constant, and time steps were 1019 and 1.0 × 1021 Pa s. This range in eclogite layer thickness ~700 yr. Topographic profi les were sampled at 22, 10, and 0 Ma is consistent with the 10–20 km eclogite root thickness that for visual comparison. occurred prior to removal during a numerical simulation of man- Following the end-member surface uplift model experiment, tle drip for the Puna Plateau (Quinteros et al., 2008). We used a we ran an eclogite root foundering experiment. The purpose of similar scale (i.e., 12.5 km) for the thickness of the eclogite root this experiment is to contrast the foreland basin response to rapid required to initiate delamination. surface uplift in the mountain belt caused by crustal thicken- Between 10 and 7 Ma, the root is removed at a linear rate ing alone against rapid surface uplift caused by a combination until it is completely removed at 7 Ma. During the period of root of foundering and crustal thickening. The model duration, time foundering, we allowed our fl exure algorithm to specify the mag- steps, and interval of topographic profi le sampling in the foun- nitude of rock uplift due to foundering and superimposed that dering experiment were the same as in the end-member surface result on the results of the rapid uplift model. Our model is kine- uplift experiment. We also applied the parameters in Table 2 and matic, so the viscous coupling between the sinking root and the the same kinematic histories in the mountain belt (e.g., shorten- overlying lithosphere is not simulated in our model. However, ing rates and propagation of deformation) as in the rapid uplift there is a possibility that viscous coupling may result in signifi - model. An eclogite root grows in our model between 25 and cant (i.e., on the order of hundreds of meters) subsidence and 10 Ma beneath the Altiplano and Eastern Cordillera back-thrust rebound (Göğüş and Pysklywec, 2008). zones, where signifi cant crustal thickening has taken place such An experiment with climate change during the late Mio- that lower-crustal rocks are subjected to pressures suffi cient cene was also conducted in order to determine its impact on the enough to produce eclogite. We assume that eclogite founder- foreland basin stratigraphy. At 9 Ma, we simulated an increase in ing is caused by a Rayleigh-Taylor instability (defi ned as the dia- mean annual precipitation by doubling both the bedrock erodibil- piric drip of a dense layer overlying a less dense layer) and that ity and sediment transport coeffi cients. Such increases could be the time scales over which the growth of the eclogite drip occur triggered by intensifi cation of the South American monsoon sys- can be calculated to within fi rst order by the results of the linear- tem and/or by an increase in orographic activity associated with stability analysis of Turcotte and Schubert (2002): the mountain belt attaining a threshold elevation. In this experi- ment, it is also necessary to increase bedrock uplift rates along 13.04μ τ = , (6) with the bedrock erodibility and sediment transport coeffi cients a ()ρ − ρ e m b because exhumation must keep pace with erosion in order for the model to reproduce the modern topography of the central Andes τ where a is the amount of time required for an instability to grow at the end of the simulation. μ by a factor of e, is the viscosity of the upper and lower layers, The bedrock erodibility (ke) of the stream power model and
and b is the original thickness of the eclogite layer. The Rayleigh- the transport coeffi cients (ks and kg) of the diffusion model have Taylor instability is not modeled explicitly, but instead we pre- both been shown theoretically to be proportional to discharge scribe a rate of eclogite root growth consistent with the observed (Paola et al., 1992; Whipple and Tucker, 1999). If we assume that spacing between foundering events in the central Andes and the mean annual discharge scales proportionally with mean annual thickness of the root required to initiate foundering. precipitation rate, then erosion rates and sediment transport rates Equation 6 is rearranged to solve for b in order to prescribe scale proportionally with mean annual precipitation rate. This is the thickness of the eclogite root required to initiate founder- reasonable assumption for erosion rates because long-term ero- ing in our model. Based on geophysical models and paleoeleva- sion rates have been shown to correlate with mean annual pre- tion proxies, the length of time over which the foundering event cipitation rates in the Andes. Sediment transport rates in alluvial occurred beneath the Altiplano was ~3 m.y. (i.e., between 10 and rivers have also been proposed to correlate with mean annual
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6.0 6.0 30.0 Ma 30.0 Ma 4.0 4.0
2.0 2.0
0.0 0.0 Elevation (km) Elevation Elevation (km) Elevation -2.0 -2.0
-4.0 -4.0
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6.0 6.0 0.0 Ma 0.0 Ma 4.0 4.0
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0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.6 2.0 3 Distance from Model Origin (103 km) Distance from Model Origin (10 km) Figure 4. The evolution of the eastern margin of the central Andes as a series of cross sections in time for both the gradual (left column) and rapid (right column) end-member uplift models. Sedimentary deposits are shaded gray, and the location of the deformation front at the time represented by the snapshot is marked by the gray vertical line.
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precipitation rates (Molnar et al., 2006). Although the magnitude ments with low erodibility contained within wedge-top basins are of increase in mean annual precipitation rate at the onset of the exhumed as the eastern portion of the Eastern Cordillera begins South American monsoon is unknown, long-term erosion rates to uplift at 30 Ma. Exhumation of the wedge-top basin deposits since the Eocene have not varied by more than a factor of two causes a period of high sediment delivery to the foredeep. compared with modern rates (Safran et al., 2005; Ege et al., 2007; By 22 Ma, rock uplift rates between 0.1 and 0.2 mm/yr Barnes et al., 2008). Thus, both erosion rates and sediment trans- cause the Eastern Cordillera to rise to a peak elevation of 3 km port rates increase by a factor of two at 10 Ma. The model dura- above msl. The increase in topographic relief causes the mean tion, time steps, and interval of topographic profi le sampling are sediment fl ux from the mountain belt into the foreland basin to the same as in the end-member uplift models. increase, such that sediment from the mountain belt outpaces foreland accommodation rates and is transported out of the fore- RESULTS deep. Subsidence rates in the back-bulge basin are an order of magnitude lower than in the foredeep. Consequently, the back- Summary of Model Outputs bulge basin is rapidly fi lled, and sediment from the mountain belt begins to bypass the fi lled back-bulge basin by 27 Ma (Fig. 5B). Topographic-profi le and sediment-fl ux time-series plots At this time, the deformation front propagates east into the Inter- show the development of the eastern margin of the central Andes andean zone and creates a step in topography located 500 km in response to the gradual uplift end-member model (Figs. 4 and from the left end of the model domain in Figure 5A. Again, there 5). Between 43 and 22 Ma, deformation is concentrated within is a period of higher-than-average sediment fl ux between 22 the Eastern Cordillera. During this period of time, topographic and 15 Ma caused by exhumation of wedge-top basins within slopes are low, and thus the sediment fl uxes from the mountain the Interandean zone. The 22 Ma snapshot represents the time belt into the Altiplano and foreland basin are also low. Sediment period when the Camargo Formation is transitioning into the eroded off of the Eastern Cordillera is not transported beyond the overlying Mondragon Formation of the Eastern Cordillera and foredeep, which is underfi lled to completely fi lled. The sediment when the Petaca Formation is being deposited in the Subandean fl ux leaving the back-bulge basin toward the east is zero at this zone. These results are also consistent with available data from time. The Subandean zone in its predeformed state is located more NW Argentina showing that the Eastern Cordillera was actively than 400 km from the deformation front near the forebulge crest deforming and eroding at ca. 18–22 Ma (Deeken et al., 2006; (Fig. 5A). Low sediment fl ux from the mountain belt is consis- Coutand et al., 2006). Similarly, thick sedimentary depocenters tent with observed paleocurrent data from the Petaca Formation, indicate high sedimentation rates in NW Argentina (Carrapa et located in the Subandean zone, showing westward paleotransport al., 2011a). from the South American craton. Between 30 and 22 Ma, there By 10 Ma, the core of the Eastern Cordillera is within 1 km is a period of higher-than-average sediment fl ux (Fig. 5B). Sedi- of its modern peak elevation, and the deformation front and
2.0 Hinterland Depositional Chaco Foreland AB 6.0
/yr) 1.6 2 m 2 1.2 4.0 0.00.0 MaMa
10.010.0 MaMa 0.8
Elevation (km) Elevation 2.0
22.022.0 MaMa Modern Front Deformation 0.4 Sediment Flux (10
0.0 0.0 0.0 0.4 0.8 1.2 1.6 2.0 40.0 30.0 20.0 10.0 0.0 Distance from model origin (103 km) Time (Ma)
Figure 5. (A) Topographic evolution of the central Andes and (B) time series of sediment supply rates into and out of the foreland basin for the gradual uplift model. The thick black lines in A represent snapshots of topography throughout the simulation, and the thick gray lines represent the maximum and mean topography for a north-south sweep of modern topography between 18°S and 21°S. The black rectangles represent the locations of the forebulge crest during each snap- shot in time, with the oldest forebulge location on the left. In B, the thick line represents the sediment fl ux leaving the back-bulge basin or right edge of the model, and the thin lines represent the sediment fl ux at the deformation front into the foredeep of the foreland basin.
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forebulge migrate another 100 km toward the South American Although rates of shortening and the propagation of the craton (Fig. 5A). As a result of the increase in relief and moun- deformation front are identical for both the gradual and rapid tain front slopes, the average sediment fl ux into the foredeep uplift models, the surface uplift and sediment fl ux responses to increases from 20 to 30 m2/yr and overfi lls the foredeep. A plot the rapid uplift model are notably different (Figs. 6A and 6B). of sediment fl ux leaving the back-bulge basin shows that at least Between 43 and 22 Ma, low rock uplift rates (i.e., 0.1–0.2 mm/yr) half of the mean sediment supply to the foreland is leaving the in the Eastern Cordillera lead to only 1.7 km of peak surface uplift back-bulge basin at this time (Fig. 5B). A signifi cant decrease over this period of time. As such, mean sediment supply rates in the amount of sediment leaving the back-bulge basin and a from the mountain belt remain low (i.e., <10 m2/yr) over the fi rst signifi cant increase in the amount of sediment entering the fore- 21 m.y. of simulation. Despite the fact that the mean sediment deep occur between 5 and 2 Ma. This is the period in time when fl ux into the foreland basin is half the rate of the gradual uplift the eastern Subandean zone is actively uplifting in both south- model, the foredeep and back-bulge basins in the rapid uplift ern Bolivia and northwestern Argentina. Thermochronologic model are able to fi ll and bypass sediment from the mountain data and growth structures (Carrapa et al., 2011b; DeCelles et belt by 25 Ma. Following an additional 12 m.y. of active uplift al., 2011) indicate that the deformation front was in the Eastern between 22 and 10 Ma, peak elevation of the Eastern Cordillera Cordillera until ca. 4 Ma and migrated out into the Subandes in the rapid uplift model rises to 2.3 km above msl, and deforma- after that. This is broadly similar to what observed in Bolivia tion propagates east into the western Subandean zone. Mean sed- (Barnes et al., 2008, 2012). High rock uplift rates in the east- iment fl ux into the foreland basin and sediment fl uxes leaving the ern Subandean zone lead to exhumation of wedge-top basins and back-bulge basin in the rapid uplift model remain low compared the development of a topographic step. Although sediment fl ux to the values for the gradual uplift model at this time. At 10 Ma, from the mountain belt is high during this time, rapid creation rock uplift rates across the mountain belt increase from 0.1 to of topographic relief in the front of the thrust belt causes high 0.3–0.5 mm/yr as the central Andes rapidly uplift in response to subsidence rates in the foredeep. A signifi cant portion of the sedi- deformation in the Subandean zone. As a result, mean sediment ment supply to the foreland basin is deposited and stored in the fl ux from the mountain belt increases from 20 to 60 m2/yr. How- foredeep. The fi nal topography at 0 Ma fi ts the average and maxi- ever, the sediment fl ux leaving the back-bulge basin decreases mum topography of the modern central Andes well. However, the shortly after the initiation of rapid uplift across the mountain belt alluvial slopes of the depositional foreland basin are a few hun- because subsidence rates in the foredeep quickly respond to sur- dred meters higher than the average channel elevations today. It face uplift within the mountain belt, trapping sediment within the is unclear if this error is due to an overprediction of the sediment foredeep near the edge of the mountain belt. The fi nal topography supply, an underprediction of transport coeffi cient, or if sediment in the rapid uplift model at 0 Ma equally reproduces the modern fl uxes at the downstream end of the model domain are too low. mean and maximum topographies of the central Andes mountain
2.0 Hinterland Depositional Chaco Foreland AB 6.0
/yr) 1.6 2 m 2 1.2 4.0 0.00.0 MaMa
0.8
Elevation (km) Elevation 2.0
10.010.0 MMaa Modern Front Deformation 0.4 22.022.0 MMaa Sediment Flux (10 0.0 0.0 0.0 0.4 0.8 1.2 1.6 2.0 40.0 30.0 20.0 10.0 0.0 Distance from model origin (103 km) Time (Ma)
Figure 6. (A) Topographic evolution of the central Andes and (B) time series of sediment supply rates into and out of the foreland basin for the rapid uplift model. The thick black lines in A represent snapshots of topography throughout the simulation, and the thick gray lines represent the maximum and mean topography for a north-south sweep of modern topography between 18°S and 21°S. The black boxes represent the locations of the forebulge crest during each snapshot in time, with the oldest forebulge location on the left. In B, the thick line represents the sediment fl ux leaving the back- bulge basin or right edge of the model, and the thin lines represent the sediment fl ux at the deformation front into the foredeep of the foreland basin.
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belt and foreland basin. Again, the predicted foreland topography of an unconformity. Sediment deposited while the eastern Sub- is slightly higher than that of the observed topography. andean depozone was located in the back bulge is completely eroded away during this time period. The shaded region between Constraints on Foreland Basin Depositional Rates 36 and 25 Ma represents the approximate depositional age for the Camargo Formation. Predicted depositional thicknesses for In both the gradual and rapid uplift models, surface uplift the Camargo Formation range between 1 and 2 km in the East- of the eastern margin of the central Andes leads to an increase ern Cordillera and Interandean depozones, but they underpre- in sediment supply and subsidence within the foreland basin. dict the >2 km thickness of the Camargo Formation observed in Figures 7A and 7B show the sediment thickness curves for four the Camargo syncline (DeCelles and Horton, 2003). By 22 Ma, depozones (i.e., the forethrust region of the Eastern Cordillera, deformation propagates into the Interandean zone. Therefore, western Interandean zone, western Subandean zone, and east- the Interandean depozone is uplifted and exhumed at this time. ern Subandean zone, respectively) within the foreland basin for Between 22 and 10 Ma, depositional rates within the western both the gradual and rapid uplift models. The solid lines repre- Subandean zone accelerate as it reaches the foredeep. At 9 Ma, a sent the results for the simulations described in the previous sec- sharp infl ection point occurs that refl ects a signifi cant increase in tion. Between 43 and 22 Ma, deformation and surface uplift are depositional rates in the eastern Subandean zone. These high dep- limited to the Eastern Cordillera. At this time, both the eastern ositional rates are inferred to be the result of high rock uplift rates region of the Eastern Cordillera and western Interandean zones in the western Subandean zone. Gradual uplift model simulations are located within the foredeep basin of the gradual uplift model were also conducted for constant and lower rock uplift rates for (Fig. 7A). Between 40 and 30 Ma, the deformation front remains the western Subandean zone. However, a slow and gradual kine- in the core of the Eastern Cordillera. A spatially fi xed load leads matic model for the deformation front could not reproduce the to constant subsidence rates and thus constant depositional rates extreme increase in isopach thickness for the Tariquía Formation in the Eastern Cordillera depozones over this period of time. given the constraints on deformation propagation rates during the The Interandean depozone, however, is located in the distal fore- late Miocene. Reproducing accurate accommodation magnitudes deep, and therefore it develops an acceleration in depositional for the foreland basin stratigraphy while also fi tting the modern rates between 31 and 25 Ma that is characteristic of an increase topography motivated us to apply an increase in rock uplift rates in subsidence rates due to an approaching mountain belt. Fur- at the front of the mountain belt relative to the gradual uplift ther to the east, the western and eastern Subandean depozones model. Rock uplift rates within the back thrust and forethrust of are located within the back-bulge basin. Low subsidence rates the Eastern Cordillera, however, remained gradual and constant. lead to low depositional rates between 43 and 36 Ma. Between Increased uplift rates between 9 and 4 Ma caused an increase in 36 and 25 Ma, the forebulge crest migrates through the west- sediment accommodation rates. As a result, a larger portion of ern Subandean depozone and leads to erosion and development the incoming sediment fl ux is stored in the foredeep depozone.
0.0 0.0
1.0 1.0
IA WSA EC 2.0 2.0 EC IA WSA 3.0 3.0 α ESA α Sediment Thickness (km) Thickness Sediment 4.0 235 km (km) Thickness Sediment 4.0 235 km ESA 150 km 150 km A 100 km B 100 km 5.0 5.0 40.0 30.0 20.0 10.0 0.0 40.0 30.0 20.0 10.0 0.0 Time (Ma) Time (Ma)
Figure 7. Uncompacted Tertiary sediment thickness for depozones located in the eastern Subandean zone (ESA), western Subandean zone (WSA), western Interandean zone (IA), and within the Eastern Cordillera forethrust region (EC) for the (A) gradual uplift and (B) rapid uplift models. The dotted, solid, and dashed lines represent simulations with fl exural parameters of 100, 150, and 235 km. Shaded regions represent the time periods during which the Tariquía Formation of the Subandean zone and Camargo Formation of the Eastern Cordillera and Interandean zone were deposited. The subsi- dence curve from Uba et al. (2006) for the location in the Subandes that we sampled is represented by black triangles in both plots.
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Extremely high depositional rates continue until ca. 4 Ma, when does not signifi cantly affect the depositional thicknesses within the basin begins to uplift and exhume. This period of time repre- the Eastern Cordillera (Fig. 7A). However, the maximum thick- sents the deposition of the Guandacay Formation and the transi- nesses of the other three depozones decrease by ~300–500 m. tion into the Emborozú Formation. Conversely, increasing the rigidity of the South American plate Although shortening rates and thrust belt propagation rates (dashed lines) to the upper limit of rigidities previously calcu- in the rapid uplift model are the same as in the gradual uplift lated for the South American plate (i.e., 4.0 × 1024 Nm) causes model, the total thicknesses of basins for the rapid uplift model the maximum depositional thickness of the three westernmost are signifi cantly different than those for the gradual uplift model depozones to increase with respect to the initial results. Chang- (Fig. 7B). Early in the model experiment, between 43 and 22 Ma, ing the rigidities of the South American plate in the rapid uplift lower rock uplift rates within the Eastern Cordillera cause depo- model has similar effects as in the gradual uplift model (Fig. 7B). sitional thicknesses within the eastern forethrust region of the Although the rigidity of the South American plate can range over Eastern Cordillera and Interandean zones to be 500 m less com- an order of magnitude, the predicted thickness of the Camargo pared to thicknesses predicted in the gradual uplift model. Both Formation within Eastern Cordillera and Interandean depozones locations develop Camargo Formation thicknesses of ~500 m, of the rapid uplift model is less than a kilometer. which is much less than the observed >1 km thickness for the Camargo Formation in the Camargo syncline. By 12 Ma, low Role of Eclogite Root Foundering on Surface Uplift and rock uplift rates in the Eastern Cordillera and Interandean zones Foreland Development lead to a factor of 2 decrease in the total sediment thickness of the western Subandean depozone compared to the thickness pre- The eclogite root foundering simulation involves the growth dicted for this depozone in the gradual uplift model. At 9 Ma, and removal of a subsurface load that modifi es the fl exural the entire mountain belt between the western Subandean zone response of the foreland basin and rock uplift distribution within and Altiplano rapidly uplifts. This rapid uplift causes an order of the mountain belt. Prior to 10 Ma, rock uplift rates and shortening magnitude increase in depositional rates within the eastern Sub- rates are prescribed to be identical to the rapid uplift model for andean depozone. Approximately 2 km of sediment are depos- the region of the mountain belt that is deforming. Between 25 and ited between 8 and 6 Ma. However, this is still 1–1.8 km less than 10 Ma, an eclogite load is allowed to uniformly grow beneath the the maximum isopach values reported by Uba et al. (2006) for Altiplano and back-thrust region of the Eastern Cordillera, where the Tariquía Formation. the deformed crust is suffi ciently thick for lower-crustal rocks to Additional simulations were conducted for both surface undergo a phase transition to eclogite (Fig. 8A). The subsurface uplift models to test the sensitivity of the sediment thickness eclogite load, the location of which is represented by the black curves to the rigidity of the South American plate. The dotted bar in Figure 8, sets up a fl exural profi le (identifi ed by the thick lines represent the results for simulations with the lowest rigid- black line) that superimposes onto the fl exural profi le caused by ity calculated for the South American plate (i.e., 1.5 × 1023 Nm). the topographic load. Although the total defl ection due to the pres- Decreasing the rigidity in the gradual uplift model simulation ence of the eclogitic root is small (i.e., ~1 m) at 25 Ma, by 10 Ma
A BT FT IA A EC IA WSES 2.0 2.0 3.0 1.5
25 Ma Deflection (km) 10 Ma Deflection (km) 2.0 1.0 1.0 1.0 Deformation Deformation Front Deformation Deformation Front 1.0 0.5 0.0 0.0 0.0 0.0 Elevation (km) Elevation (km) Elevation -1.0 AB-1.0 -1.0 -0.5 0.0 0.4 0.8 1.2 1.6 2.0 0.0 0.4 0.8 1.2 1.6 2.0 Distance from the model origin (103 km) Distance from the model origin (103 km)
Figure 8 (A and B). Topographic and fl exural profi le snapshots in time for the eclogite delamination model. The shaded gray regions represent Tertiary sedimentary deposits, and the pre-Tertiary bedrock is white. Lithospheric defl ection caused by accumulation and delamination of an eclogite load is represented by a thick black line, the scale of which is shown on the right vertical axis, and the position of the eclogite load is represented by the black horizontal bars. The locations of the tectonomorphic zones are represented by the following labels: A—Altiplano; EC—Eastern Cordillera; BT—Eastern Cordil- lera back thrust; FT—Eastern Cordillera forethrust; IA—Interandean; WS—western Subandean; ES—eastern Subandean.
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the total defl ection is on the order of 1 km at the center of the of the rapid uplift model. Sediment supply rates are low until eclogite root. At 25 Ma, the edge of the eclogite load is located ca. 10 Ma, when the eclogite drip event accompanied by the ~150 km from the current deformation front. As a result of the dis- rapid uplift of the eastern Interandean and western Subandean tance between the load and deformation front, subsidence caused zones lead to increased channel slopes in the front of the moun- by the eclogite load is small in the foredeep, and nearby to the tain belt (Fig. 9A). The amount of sediment bypassing the fore- east, subsidence changes to rock uplift in the distal foredeep. As land basin in the eclogite root foundering model is also similar to time progresses, the Interandean and Subandean depozones are the rates predicted by the rapid uplift model, with the exception subject to an additional component of rock uplift because they are of a greater magnitude of sediment fl ux between 15 and 5 Ma. located on the fl exural forebulge related to the eclogite root load. This increase in sediment leaving the foreland per time period is Following 10 Ma, the polarity of lithospheric defl ection reverses either the result of increased erosion rates in the mountain belt or as the eclogite root is removed (Fig. 8B). The solid black curve decreased sediment accommodation rates in the foreland basin. is the total amount of defl ection that occurs over 3 m.y. as the A comparison of sediment thicknesses predicted by the rapid subsurface load is removed. A signifi cant amount of rock uplift uplift and eclogite root foundering models through time for the occurs directly over the center of the load in the eastern Altiplano Interandean, western Subandean, and eastern Subandean zones and western back-thrust region of the Eastern Cordillera and rap- shows that the drip event modifi ed sediment accommodation by idly decays into the core of the Eastern Cordillera. Further east, less than 10% of the maximum basin thicknesses before founder- the Interandean and western Subandean zones experience subsi- ing (Fig. 9B). The western Interandean zone basin deposits are dence on the order of meters to tens of meters. Beyond the western ~100 m thicker at 22 Ma for the eclogite root foundering model Subandean zone, the defl ection due the eclogite foundering is less (solid line) compared to the rapid uplift model (dashed line). than a meter. Rock uplift rates in the Altiplano and the core of Between 25 and 22 Ma, the western Interandean zone is located the Eastern Cordillera due to the eclogite root foundering range close enough to the edge of the eclogite load to experience sub- between 3.0 and 6.0 × 10−4 m/yr. Maximum defl ection rates in sidence, which causes the change in sediment thickness between the eastern Interandean and western Subandean zones are on the models. Conversely, the Subandean basins are located far enough order of 10−7 and 10−5 m/yr. Rock uplift rates near the mountain from the eclogite load at that time to undergo rock uplift. The front and within the foreland basin (i.e., Interandean and Suban- western Subandean basin is the least affected basin by the drip dean zones) prescribed in the rapid uplift model at 10 Ma are on event because of its proximity to the infl ection point between the order of 6.0–5.0 × 10−4 m/yr, and, therefore, the fl exural signal subsidence and rock uplift caused by accumulation of eclogite. from a delamination event is small when compared to the sub- The eastern Subandean zone also experiences rock uplift due to sidence or rock uplift due to crustal thickening. Conversely, rock foundering, and, thus, lesser sediment thicknesses between 20 uplift rates due to the delamination are slightly greater in the Alti- and 10 Ma. Decreased accommodation in the Subandean zones plano and the back-thrust region of the Eastern Cordillera than the leads to an increase in sediment leaving the basin prior to 10 Ma. prescribed uplift rates due to crustal thickening. Following 10 Ma, the drip event leads to minor subsidence in the Sediment supply into the foredeep predicted by the eclo- eastern Subandean zone, and the sediment thickness difference gite root foundering model shows a similar behavior to that between the two models becomes negligible.
40.0 2.0 0.0 /yr) 2 Mountain Front (10 Sediment Flux from the 1.0 30.0 1.5 IA WSA EC 2.0 1.0 20.0 3.0 2 10.0 0.5 m 4.0 2
/yr) ESA
Sediment Thickness (km) Thickness Sediment 5.0
Bypassed Sediment Flux (m AB 0.0 0.0 40.0 30.0 20.0 10.0 0.0 40.0 30.0 20.0 10.0 0.0 Time (Ma) Time (Ma)
Figure 9. (A) Time-series data for sediment fl uxes into and out of the foreland basin and (B) uncompacted sediment thick- ness for depozones located in the eastern Subandean (ESA), western Subandean (WSA), Interandean (IA), and Eastern Cordillera (EC) tectonomorphic zones for the eclogite delamination model. The black symbol at the top of A and the bot- tom of B represents the growth and delamination of the eclogite root between 25 and 10 Ma. In A and B, the thick lines represent the eclogite delamination model, and the thin dashed lines represent the rapid uplift model.
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40.0 2.0
/yr) rapid uplift model (Fig. 10). The effect of lowering both erosion 2 Mountain Front (10 Sediment Flux from the and sediment transport coeffi cients by a factor of two early on 30.0 1.5 in the simulation is a few-million-year delay in the time when sediments begin to exit the back-bulge basin. Decreasing both erosion rates and sediment transport rates also has a cumulative 20.0 1.0 effect of decreasing sediment bypass rates (solid black line) by
2 almost half the value of sediment bypass predicted by the rapid 10.0 0.5 m
2 uplift model (dashed line). Following 9 Ma, the sediment bypass /yr) rates suddenly increase by a factor of 1.5 times the previous fl ux.
Bypassed Sediment Flux (m 0.0 0.0 An abrupt change in erosion rates and transport effi ciency 40.0 30.0 20.0 10.0 0.0 Time (Ma) due to climate change should be expected to have an effect on grain size within the depositional basin, and, therefore, we tracked Figure 10. Sediment fl ux time-series data for the climate the gravel-sand interface for each of the experiments (Fig. 11). A change model. The thick line represents the sediment fl ux comparison of the gradual uplift, rapid uplift, and climate change leaving the back-bulge basin during the climate change model, and the dashed lines represent the sediment fl ux model results (Fig. 11) reveals that in general the gravel-sand leaving the back-bulge basin during the rapid uplift mod- boundary closely tracks the location of the deformation front el. The dashed vertical line represents the timing of the (shown as the dashed line). An exception to this observation onset of the South American monsoon. occurs immediately following times when the deformation front propagates basinward, and the gravel-sand interface lags behind the deformation front for ~1–2 m.y. until the regional slope of Role of Climate Change in Foreland Basin Development the newly formed wedge-top basin increases above the threshold slope for gravel transport. Once gravel reaches the deformation The results for the climate change experiment (i.e., a dou- front, gravel progradation into the foredeep appears to be lim- bling of the bedrock erodibility and sediment transport coeffi - ited within a narrow zone of ~50 km from the deformation front. cients in late Miocene time) are reported here. Bedrock uplift rates Figure 11A compares the gravel-sand boundaries for the gradual during the late Miocene within the actively deforming mountain and rapid uplift models. Prior to 10 Ma, the mean location of the belt in this simulation are increased with respect to those of the gravel-sand boundaries for both models generally remain in front rapid uplift model to maintain similar surface uplift histories, of the deformation front following the 2 m.y. lag periods. How- and, thus, the basin accommodation histories between the rapid ever, the initiation of rapid uplift in the Interandean and western uplift model and the climate change model are the same. We only Subandean zones at ca. 10 Ma causes the gravel-sand bound- report observations for sediment fl ux and grain size. Although ary to retrograde back into the wedge-top zone as accommoda- erosion rates and sediment transport rates are a factor of two tion rates exceed sediment supply rates. Both models appear to lower for the fi rst 35 m.y., the fi rst-order behavior of sediment closely overlap each other, with two exceptions. Between 22 and fl ux entering the basin appears to be unchanged from that of the 15 Ma, the gravel front in the gradual uplift model progrades
25.0 25.0 A B 20.0 20.0
15.0 Foredeep 15.0 Foredeep
10.0 Hinterland 10.0 Hinterland Time (Ma) Time (Ma)
5.0 5.0
0.0 0.0 0.0 200.0 400.0 600.0 0.0 200.0 400.0 600.0 Distance from Model Origin (km) Distance from Model Origin (km)
Figure 11. Gravel-sand interface time-series data for the (A) gradual and rapid uplift models and the (B) rapid uplift and climate change models. In A, the gradual and rapid uplift models are represented by the solid and dotted lines, respec- tively, and in B, the climate change and rapid models are represented by the solid and dotted lines, respectively. The location of the deformation front is shown by the dashed line in both plots.
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more rapidly than the gravel front of the rapid uplift model due center of the load. As such, a comparison of the sediment thick- to a combination of greater uplift rates in the Interandean zone ness time-series results for the gradual and rapid uplift models wedge-top basin and greater sediment supply rates. Between 8 shows little change in accommodation rates following 10 Ma and 5 Ma, the mean location of the gravel-sand interface of the in the eastern Subandean zone, even though the peak elevations rapid uplift model retrogrades more than the gravel-sand inter- of the Eastern Cordillera differ by 2 km (Fig. 7). Although the face of the gradual uplift model due to greater subsidence rates. difference in peak elevation within the Eastern Cordillera does Figure 11B compares the results for the climate change model to not strongly affect late Miocene foreland basin sediment accom- the rapid uplift model and, thus, should highlight the effects of modation, we can constrain the early surface uplift history of changes in precipitation on grain size. Although both the bedrock the Eastern Cordillera and Altiplano with the middle Cenozoic erodibilities and transport coeffi cients are a factor of two less in stratigraphy of the Eastern Cordillera and western Interandean the climate change model compared to the rapid uplift model zones because these depozones were located at least 100 km between 25 and 9 Ma, the gravel-sand boundaries generally track closer to the Eastern Cordillera than the central Subandean zone. each other well through time. However, there are a few brief peri- DeCelles and Horton (2003) measured a thickness of over 2 km ods of time when the gravel-sand interface of the climate change for the Camargo Formation within the Camargo syncline of the model lags behind that of the rapid uplift mode. When the bed- Eastern Cordillera. A comparison of sediment thicknesses depos- rock erodibilities and transport coeffi cients increase by a factor ited between 36 and 22 Ma predicted by the gradual and rapid of root 2, the results of the climate change model do not vary uplift models shows that a sediment accommodation thickness signifi cantly from the trends of the rapid uplift model. of 2 km within the early foreland basin is not achievable unless rock uplift rates in the core of the Eastern Cordillera more closely DISCUSSION resembled the rock uplift rates specifi ed within the gradual uplift model (Fig. 7). Although there is uncertainty in the depositional Two end-member surface uplift models have been proposed age of the Camargo Formation, the rapid uplift model could not for the central Andes (Garzione et al., 2008). One model involves create suffi cient sediment accommodation space if deposition a long history of constant deformation since the late Eocene, began as early as 40 Ma. If the erosion rates in our model are accompanied by a gradual increase in mean surface elevation close to the effective erosion rates during the early Miocene and prior to 10 Ma. The opposing end-member model suggests that late Oligocene, then the thick deposits of the Camargo Formation the central Andes did not gain signifi cant elevation until after would imply that the peak elevation of the Eastern Cordillera was 10 Ma. Our results show that if the elastic properties of the under- between 2 and 3 km by 22 Ma. thrusted South American plate remain effectively constant, then An Eastern Cordillera peak elevation of 2–3 km might rapid rock uplift is likely to have occurred within the Interandean have acted as an effective topographic barrier to moisture that and western Subandean zones during the late Miocene in order to was being transported west across the central Andes. Today, the generate suffi cient accommodation space for the Tariquía Forma- topography of the eastern fl ank of the central Andean Plateau pre- tion observed in outcrops within the central to eastern Subandean vents a signifi cant amount of moisture that originates from the zone (Fig. 7). Constraints on the initiation of rapid rock uplift in Atlantic Ocean and Amazon Basin from being transported west the Interandean and Subandean zones are based on the isopach into the Altiplano and Western Cordillera by the South Ameri- data for the Yecua and Petaca Formations. If rock uplift rates were can summer monsoon (Strecker et al., 2007). A similar behav- greater in the Interandean and Subandean zones prior to 9–8 Ma, ior occurs in the southern Andes, where the moisture from the when the Tariquía Formation began to be deposited, then the iso- Pacifi c Ocean carried by the Southern Hemisphere westerlies pach trends for the Yecua and Petaca Formations observed within rains out on the west coast of South America and the Patagonian the central Subandean zone would show maximum thicknesses Andes, leading to semiarid conditions on the eastern fl ank of the greater than 825 m. Rock uplift rates prescribed for the Interan- Andes. The southern Andes are an effective moisture barrier even dean and western Subandean zones in the gradual uplift model though their mean elevation is around 2 km lower than the central lead to maximum thicknesses for the Petaca-Yecua deposits that Andes, with peak elevations ranging from 2 to 3 km between are on the order of 1 km. Although other studies have proposed 38°S and 50°S. We envision the topography of the central Andes that decreasing the fl exural rigidity of the South American litho- during the early to middle Miocene as being similar to the south- sphere can explain an increase in accommodation for the Tariquía ern Andes today, which suggests that the orogen may have been Formation (Prezzi et al., 2009), we show that a constant fl exural an effective barrier to moisture being transported to the southwest rigidity model can also fi t the late Miocene isopachs well. from the Atlantic. If this is the case, then basins within the Alti- At 10 Ma in the models, both the Altiplano and Eastern plano and Western Cordillera regions of the central Andes should Cordillera are suffi ciently far from the central and eastern Sub- become increasingly arid around or soon after 22 Ma. Based on andean zones such that their contribution to the accommodation abrupt changes in lacustrine facies within basins, the onset of space is low compared to that of the Interandean and western hyperaridity has been documented to have occurred between 10 Subandean zones. The defl ection of an elastic beam in response and 6 Ma for basins within the Western Cordillera between 18°S to a point load decreases exponentially with distance from the and 22°S (Gaupp et al., 1999; Sáez et al., 1999). However, based
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on oxygen isotopes, soil morphological characteristics, and salt would be more predominately infl uenced by the central Andes chemistry, Rech et al. (2006) and Rech et al. (2010) proposed would be the deposits of deep-water fans offshore of the Rio del that the onset of hyperaridity could have occurred in the Atacama la Plata estuary, which samples the Andes between 18°S and Desert earlier, between 19 and 13 Ma. In northern Argentina 34°S. To our knowledge, no analysis has been conducted for the between 25°S and 26°S, recent thermochronologic data from late Cenozoic sediments of the Rio del la Plata fans. the Puna Plateau show that the Eastern Cordillera was deformed A hypothetical eclogite foundering event in the eastern Alti- and exhumed at the same time as the Eastern Cordillera further plano region during middle Miocene to Pliocene time was syn- to the north in southern Bolivia (Carrapa et al., 2011a, 2011b). chronous with increased grain size and depositional rate in the Interestingly, Vandervoort et al. (1995) also documented an ear- foreland basin stratigraphy. Our results show that the distance lier shift from nonevaporitic to evaporitic sedimentary deposits from the center of mass of the load is the most important parame- within Puna Plateau basins located between 24°S and 26°S at ter for determining how the growth and removal of a lower-crustal ca. 15 Ma. A middle Miocene onset of aridity-hyperaridity within load would affect rock uplift and subsidence within the mountain the Andean Plateau (perhaps as far south as the Puna) and basins belt and foreland basin (Fig. 8). An infl ection point between sub- on the western margin of the central Andes would be consistent sidence and uplift occurs at a distance of (π/2)α (i.e., ~235 km with the peak elevations within the Eastern Cordillera that were for this study) from the growing eclogite root load. During the necessary to generate suffi cient accommodation space for the Oligocene to early Miocene, the foredeep depozone was located Camargo Formation. closer to the eclogite root than it would be for the remainder of Another output of our numerical model during the end- the simulation (i.e., less than [π/2]α km). As a result, the accu- member uplift model experiment was a time series for sediment mulation of eclogite drove an additional 100 m of subsidence, leaving the foreland basin (Figs. 5 and 6). Changes in the sedi- which is more than a factor of 2 greater than subsidence added ment bypass rates would not be directly recorded in the foreland to the western and eastern Subandean depositional basins during basin stratigraphy, and, therefore, one must look at the stratig- the late Miocene. The greatest defl ection occurred directly above raphy of adjacent intracratonic basins or the continental shelf to the center of the eclogite root, which was located in the eastern directly sample this signal. Presently, a topographic divide exists Altiplano and westernmost Eastern Cordillera. This defl ection in southern Bolivia that splits the fl ow of major rivers draining off led to ~0.5–1 km of rock uplift in the Altiplano. Therefore, an the central Andes north into the Amazon and southeast into the additional 2 km of surface uplift due to crustal thickening and Rio del La Plata cratonic basins. Based on the sediment bypass sediment deposition are required to achieve the modern mean time series for each of the simulations, the sediment fl ux compo- elevation of the Altiplano if it was located at 1 km around 10 Ma. nent from the central Andes should steadily increase from the late Similar amounts of rock uplift due to crustal thickening (i.e., 1– Oligocene into the Quaternary, with a local minimum between 8 2 km) are required in the Eastern Cordillera to reach its modern and 3 Ma. Analysis of drill-core sediments from the Amazon fan mean and spatially averaged maximum elevations. However, less and Ceara Rise show that Andean-derived sediments reached the rock uplift due to crustal thickening is necessary if the average continental margin between 16.5 and 11.3 Ma (Dobson et al., thickness of the eclogite root was signifi cantly larger than the 2001; Figueiredo et al., 2009). Between 9 and 6.8 Ma, sedimen- value applied in this study. We infer that this is less likely because tation rates increased for both the Amazon fan and Ceara Rise. a thicker eclogite root would grow a signifi cantly sized instability This increase in sedimentation rates was interpreted as the estab- in less than a period of 3 m.y., which is the proposed length for lishment of a transcontinental Amazon drainage system that fully the delamination period. Our results also show that <0.5 km of linked the Andean forelands to the Amazon fan. Sedimentation rock uplift is contributed to the eastern edge of the Eastern Cor- rates continued to increase into the Pliocene–Pleistocene during dillera and western Interandean zones by eclogite removal. Barke the period of most rapid uplift within the northern Andes (Hoorn and Lamb (2006) calculated that the San Juan del Oro paleosur- et al., 1995). An overall increase in sedimentation rates on the face, which overlies the eastern part of the Eastern Cordillera, Atlantic shelf is predicted by our model. The higher the mean was uniformly uplifted by ~1 km. Therefore, the eclogite root elevation of the mountain belt, the more the sediment supply may must be located closer to or beneath the forethrust of the Eastern exceed available accommodation space. Our model also predicts Cordillera to uniformly uplift it by 1 km. However, tomography a factor of two increase in sediment fl ux leaving the foreland data suggest that foundering is more likely beneath the Altiplano basin during the late Pliocene to Pleistocene as foredeep accom- and western part of the Eastern Cordillera (Beck and Zandt, modation rates decrease in response to slower rock uplift rates in 2002). Thus, part of the 1 km of rock uplift of the San Juan del the Interandean and Subandean zones. One aspect of our model Oro surface is likely the result of crustal thickening. results that is not recorded in the Amazon fan is a late Miocene Climate change was the fi nal process that we tested for the to early Pliocene decrease in sediment supply rate. The absence foreland basin of the central Andes of southern Bolivia. Erosion of a drop in sedimentation rates may be expected because a small rates and transport coeffi cients between 43 and 9 Ma were a fac- portion of the Amazon Basin drains the central Andes, and, there- tor of 2 less than the values between 9 and 0 Ma. As a direct fore, sediment supply to the Amazon fan is more predominantly result, peak sediment fl ux into the foreland basin and sediment infl uenced by the northern Andes. A stratigraphic data set that bypass rates were signifi cantly less than the values resulting from
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the rapid uplift model (Fig. 10). At the onset of the South Ameri- Further crustal thickening during the middle Miocene may have can monsoon, both sediment bypass rates and sediment fl ux rates increased mean topography of the Eastern Cordillera to the point into the foredeep increased signifi cantly. The increase in sedi- where it became an effective barrier to easterly moisture derived ment supply to the foredeep is predominantly controlled by the from the Atlantic (as shown in Fig. 5A); in turn, this initiated rapid uplift of the mountain belt instead of by the increase in aridity in the Western Cordillera and Atacama Desert regions at mean annual precipitation. Increasing erosion rates and decreas- that time. Our results suggest that peak topography of the Eastern ing transport rates should lead to steeper slopes in the proximal Cordillera was well above 3 km prior to 10 Ma and, therefore, foreland basin. Both sediment supply and transport capability do not support the rapid uplift model, which predicts ~2 km of increase as mean annual precipitation increases, which leads to late Miocene rapid surface uplift within that region. However, minimal change in the regional slopes for the foreland basin, as our results do support rapid rock and surface uplift within the these effects cancel each other. Regardless, regional slopes in the Interandean and western Subandean zones to produce the amount foreland basin component of the model appear to be predomi- of accommodation required to store the thick Tariquía Formation, nately affected by rapid subsidence due to crustal thickening which was deposited within a period of 2 m.y. near the deformation front instead of by increasing precipitation. Our results also support the hypothesis that the fi rst-order Another way to gauge the effect of climate change is to track trends in the Cenozoic foreland basin stratigraphy of the Suban- the boundary between gravel and sand deposition. Paola et al. dean zone were predominantly infl uenced by its distance from (1992) demonstrated that sinusoidal variations in both sediment the approaching mountain belt. An increase in topographic loads supply and transport coeffi cients lead to migration of the gravel- located less than (3/4)πα km from a depozone can cause defl ec- sand interface, especially if the forcing period is small compared tions up to the order of a kilometer. Beyond this distance, defl ec- to the basin equilibrium time. Our results show that the fi rst-order tion ranges up to hundreds of meters near the forebulge. Dur- migration of the gravel-sand boundary appears to be unaffected ing the late Miocene, the eclogite root was located more than a by the factor of 2 increase in both of these parameters over the distance of (3/4)πα from the Subandean depozone. As such, the time scales that we are sampling (Fig. 11). Based on our results, accommodation rates within the central-eastern Subandean zone the threshold slope term appears to be a more primary control foredeep were positive, but only on the order of tens of meters. on gravel progradation than the transport coeffi cient term. Rock Therefore, the defl ection caused by an approaching mountain belt uplift in the hinterland leads to steeper slopes that are capable of exceeded the defl ection caused by eclogite delamination. Also transporting gravel. As a result, gravel trapped near the moun- based on our results, the factor-of-fi ve increase in depositional tain front can rapidly prograde toward the new deformation front rates was not likely the result of an increase in erosion rates and when it is located in a wedge-top basin. Eventually, rock uplift sediment transport rates caused by climate change. Increased ero- leads to the exhumation of pre-Cenozoic units near the defor- sion rates would lead to isostatic rebound of the mountain front, mation front, which can produce gravel during bedrock incision. and thus decreased sediment accommodation within the foredeep, Conversely, the regional slope within the foreland is insuffi cient if crustal thickening does not increase as well. If crustal thicken- to transport gravels far into the foredeep. Therefore, a combina- ing does increase with erosion rates, as in our model, then the tion of processes causes progradation of the gravel-sand interface additional sediment supplied bypasses the foreland basin because during forward propagation of the deformation front; climate it is already overfi lled. In addition to accommodation rates, the change appears to have played a secondary role in controlling location of the deformation front in time appears to exert a pri- gravel progradation for the central Andes. mary control on the gravel-sand interface. Increasing erosion rates and sediment transport rates by a factor of 2 due to the onset of the CONCLUSIONS South American monsoon at 9 Ma did not generate a long-term progradation in the gravel-sand interface. Instead, the gravel front Based on our modeling results, we propose that the early retreated toward the deformation front due to the rapid subsidence surface uplift history (i.e., prior to 22 Ma) of the eastern margin rates within the foredeep basin. Actively uplifting fold-and-thrust of the central Andes more closely resembled the gradual uplift belts are able to achieve channel slopes that are above the thresh- model. When the rigidity of the South American plate ranged old for gravel entrainment and produce gravel by bedrock inci- between 1.5 × 1023 and 4.0 × 1024 Nm, surface uplift of the core of sion. As a direct result, the gravel-sand interface rapidly progrades the Eastern Cordillera in the rapid uplift model produced grossly when the deformation front propagates into the foreland basin. undermatched sediment accommodation during the deposi- tion of the >2-km-thick Camargo Formation. The gradual uplift ACKNOWLEDGMENTS model more closely fi ts the observed sediment thicknesses for the Camargo Formation located in the Eastern Cordillera, and, thus, Our research was supported in part by ExxonMobil funding. We the core of the Eastern Cordillera experienced rock uplift rates thank Peter DeCelles and Joellen Russell for their helpful com- that would have led to signifi cant topography (i.e., peak eleva- ments during the course of the work. We also thank Barbara Car- tions near 2–3 km as shown in Fig. 5A) by the early Miocene rapa, Chris Poulsen, and Chris Beaumont for constructive reviews if basin-averaged erosion rates were on the order of 10−4 m/yr. that improved the quality of the presentation signifi cantly.
Downloaded from https://pubs.geoscienceworld.org/books/chapter-pdf/963891/mwr212-17.pdf by University of Arizona user on 27 August 2019 356 Engelder and Pelletier
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